In the forming sheets of material, such as plastics, paper or other web-based products, the sheet is generally formed in a continuous process as a moving film or web. In order to most effectively monitor and control the process, one or more properties of the web must be determined while the web is in motion. The properties of interest include web basis weight, which is the weight per unit area of the web.
To measure the desired web property, one or more sheet sensors are employed in fixed or web traversing structures. The moving web can be constrained to move over a fixed reference surface, but more commonly the moving web is unsupported in the measurement region and is subject to deviations from the nominal path through the measurement region. The nominal path through the measurement region is usually referred to as a “pass-line”, and deviations from that pass-line such as “flutter” or other lower frequency deviations alter the pass-line of the web through the measurement region.
A desirable method of forming a measurement region across the width of the web is to utilize nuclear radiation in a traveling sensor arrangement that traverses the moving web to measure the desired web property. A radioactive thickness/density gauge is based upon the principle that a mass of material will absorb the products of radioactive emission in a known and repeatable manner. An industrial web-gauging instrument, known as a beta-gauge, typically utilizes a radioactive isotope that decays through beta particle emission. The radioactive isotope is mounted in an enclosed head or source, which projects the radiation through the web to a second head that includes a radiation detector. The amount of radiation sensed by the detector is directly related to the amount of radiation absorbed by the web material being measured.
The relative position of the moving web in the measurement region space or gap between the source and detector heads is called the web pass-line. Due to the nature of beta particle interaction with the web material, specifically due to the angular dispersion of the beta particles as they pass through the web, the amount of radiation sensed by the detector varies with pass-line deviations. Since the fluctuating web pass-line can vary significantly and at a high frequency during a measurement of the moving web, the accuracy of the gauge or sensor in part depends on the ability to compensate for pass-line variations. In particular, basis weight sensors (especially nuclear-based ones) are passline-sensitive. Error in measurement is induced when the sheet moves up and down in the gap.